drm/panfrost: Fix a deadlock between the shrinker and madvise path

panfrost_ioctl_madvise() and panfrost_gem_purge() acquire the mappings
and shmem locks in different orders, thus leading to a potential
the mappings lock first.

Fixes: bdefca2d ("drm/panfrost: Add the panfrost_gem_mapping concept")
Cc: <stable@vger.kernel.org>
Cc: Christian Hewitt <christianshewitt@gmail.com>
Reported-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201101174016.839110-1-boris.brezillon@collabora.com